The invention relates to a method and an arrangement for measuring the structures of an object with the help of a scanning element that is allocated to a coordinate measuring device and that starts from a flexible shaft, with the scanning element touching the object and with its position being then determined directly or indirectly with at least one reticule, which is allocated to the scanning element, with an optical sensor.
An arrangement of the kind described above is already known (DE 297 10 242 U1). In this familiar arrangement, surface topography of an item or object is measured with a photogrammetry system and the scanning element. The scanning element, e.g. a ball, is arranged at the end of an elastic shaft. In the shaft reticules can be incorporated, whose positions are recorded by the photogrammetry system relative to a tracer reference system. The position of the scanning element is determined e.g. by the reticule positions.
We also know of a measuring system for recording surface topography of objects with a data transmitter, which consists of a tracer pin and a light source of a defined kind at its end. The light source is guided along the outline that is to be recorded. In this method, an optical receiver records the respective position of a light source, which forms a luminous spot or light spot, in a three dimensional Cartesian coordinate system. A computer evaluates the measurement results. As an extension of the tracer pin, the light source takes on the shape of e.g. a concentrically enclosed glass fiber (DE 40 02 043 C2).
We furthermore know of a scanning system for measuring small structures, which is based on quartz crystal that stimulates a glass fiber with scanning element. When it touches the surface of the item, the dampening of the system is evaluated. Although this technique enables small scanning forces, it is subject to a relatively high degree of inaccuracy (measurement error 5 xcexcm).
And finally, for the purpose of measuring structures, we are familiar with the method of determining the position of a scanning element with the help of a microscope; for this method, a transmission procedure must be applied due to the equipment so that only structures of through-holes or other perforations can be measured.
The invention at issue is based on the problem of developing a method and a device for measuring surface topography of objects with which any random structure and object of varying surface hardness can be determined with a high degree of measurement exactness.
According to the invention, a method for measuring the structures of an object solves the problem with a scanning element that is allocated to a coordinate measuring device and that starts from a flexible shaft and touches the object and whose position is then determined directly or indirectly with at least one reticule, which is allocated to the scanning element, with a sensor in such a way that, with the exception of a free bending length comprising the scanning element and/or the reticule, the shaft runs within a rigid or basically rigid guiding piece and that the scanning force, which occurs upon contact between the scanning element and the object, is determined from the excursion of the scanning element and/or reticule from a neutral position. In doing so, particularly the scanning force is set to a value that has been adjusted to the properties of the object, due to the specification of the bending length. This can occur through shifting of the shaft within the guiding piece. The scanning force values obtained this way can then be taken into consideration in subsequent measurements of the object""s structures.
In the case of some objects, the scanning force has considerable influence on the measuring results. In the invented method, it is possible to adjust the scanning force to the properties of the object, such as surface topography and surface hardness, as a parameter of the measurement.
In a preferred version, the scanning force of the scanning element is determined based on the following equation:   F  =            3      ·      E      ·      f      ·      I              1      3      
wherein F describes the scanning force, E the modulus of elasticity of the shaft, l the effective bending length of the shaft between the rigid guiding piece and the scanning element, I the axial surface moment of the shaft, and f the excursion of the latch element from a neutral position. The modulus of elasticity, axial surface or inertia moment and length are specified by the design or material properties of the device and can be summarized into one constant. This makes the scanning force proportional to excursion and it can be determined quickly and without extensive arithmetical operations and time.
If necessary, the shaft can be moved within the guiding piece in order to modify the effective bending length. Apart from this, the rigid guidance of the shaft ensures in a constantly reproducible manner that the shaft has a defined bending length.
In a useful version, the scanning force is adjusted as a controlled variable in a control circuit to a specifiable constant or nearly constant value, with the support of at least one motor being moveable in the form of an actuator. The scanning force in this version can be maintained at a specified value throughout the entire measuring process of the structure. For measuring the surface topography in three dimensions, it has frequently proven useful if the support can be moved by drives in five degrees of freedom with the elastic shaft connected to it. For this purpose, numerical control circuits are suited. In particular, the support is connected with the adjusting mechanism for the optical system into one unit, which can be adjusted with a motor into five degrees of freedom.
Furthermore it is beneficial when the scanning element""s excursion is measured with an optical sensor, which records the difference between the scanning element""s position when it is in neutral position and the scanning element""s position when it touches the surface of the object. It is useful if the sensor, which is the same as the one used for measuring the structure, moves together with the support.
The position of the scanning element and/or of the at least one reticule is determined optically especially with reflective radiation and/or radiation shutting off the element or reticule and/or reflecting from the scanning element. It is useful if the tracer extension or the shaft are a fiber-optic light guide or comprise such a light guide in order to feed the required light to the scanning element or the reticule via this light guide.
It is also possible that the scanning element and/or the reticule have the design of a self-luminescent electronic element such as LED or comprise such a component.
In particular, the invention excels through an arrangement for measuring the structures of objects with the help of a scanning element that is allocated to a coordinate measuring device and that starts from a flexible shaft and can be brought into contact with the object and whose position can then be determined directly or indirectly with at least one reticule, which is allocated to the scanning element, with a sensor, with the arrangement excelling through the fact that the shaft, with the exception of a free bending length comprising the scanning element and/or the reticule, runs in a rigid or basically rigid guiding piece. This specifies the scanning force, defined by the effective bending length, which makes the arrangement adjustable to the conditions of the surface properties of the object that is to be measured.
In particular, the arrangement excels through the fact that a sensor is provided for measuring the excursion of the scanning element and/or of the reticule from a neutral position, that the shaftxe2x80x94with the exception of a free bending length comprising the scanning element and/or the reticulexe2x80x94is run in a rigid or basically rigid guiding piece, that the guiding piece can be moved relative to the object""s surface together with the shaft with at least one motor and that with an evaluating device the scanning force can be determined from the excursion of the scanning element and/or the reticule from the neutral position. With this device, the scanning force of the scanning element can be adjusted to the conditions of the surface properties of the object. For example, the scanning force can thus be adjusted to the hardness properties of the surface, achieving as high a degree of measuring accuracy as possible. This allows the possibility of arranging the shaft in a moveable fashion within the guiding piece.
In particular, the arrangement determines the scanning force from the measurement value of the scanning element""s excursion based on the following equation:   F  =            3      ·      E      ·      f      ·      I              1      3      
wherein F describes the scanning force, f the excursion, l the length of the shaft in its effective bending length, E the modulus of elasticity of the shaft and I the axial surface moment of the shaft. Since all values in this equation except for the excursion f and the scanning force are constant, the excursion can be calibrated while taking the constant in the unit of the scanning force into consideration.
In a useful version, an optical system is provided for recording the scanning element""s or the reticule""s excursion from a neutral position, and it can be moved as a unit with at least the scanning element and the shaft. The system, with which particularly also surface topography is determined, allows determination of the neutral position taken on by the scanning element and/or by the at least one reticule or several reticules if no contact with the object or item occurs. The appropriate geometrical position of the scanning element or of the at least one reticule can be memorized and appropriately adjusted when the system changes its position. When the scanning element touches a surface of the object, the difference between the neutral position and the position upon contact is recorded, e.g. from the scanning element""s position and the allocated neutral position. In particular, the optical system for recording the scanning element""s or reticule""s excursion is arranged as an actual value transmitter in a control circuit, whose controlled variable is the scanning force and which, as the actuator, comprises at least one motor for movement of the unit, which includes the scanning element, the shaft and the sensor. The scanning element can be attached to the shaft by way of gluing, welding or other suitable fastening procedures. The scanning element and/or the reticule itself can also be a section of the tracer extension.
At the ends, the shaft itself can have the design of a tracer or comprise a tracer. In particular, the scanning element and/or the reticule can be connected with the tracer extension as well as the shaft in an exchangeable manner.
In order to be able to determine nearly any random structure, the invention furthermore incorporates the feature that the scanning element with the shaft and the optical system can be adjusted by a support that can be set in five degrees of freedom. The support itself in turn can form a unit with the sensor and/or be connected with the sensor. In a preferred version, the scanning element and the shaft represent a fiber-optic light guide, with the scanning element being fed light via the light guide. In this version, the scanning element, which is recorded as a luminous spot or light spot by the optical system, radiates. The scanning element and/or the at least one reticule can also have a reflective design. However, there is also the possibility of designing the scanning element and/or the reticule as a self-luminescent element such as LED.
In particular, the optical system, which is employed to detect the excursion of the scanning element from its neutral position and for measuring structure, is an electronic camera. Measurement of the excursion is performed in particular also with a focus system, as is the one already known in optical coordinate technology for focusing on the object surface. In this version, the contrasting function of the image is evaluated in the electronic camera.
For the purpose of determining the structure of objects, the scanning element""s position is measured directly. Many different physical principles are possible for this direct measuring procedure. Since spatial measurement of the scanning element""s excursion in a large measuring range must be very precise, e.g. in order to enable continuous scanning processes, and in order to absorb excessive object scanning (e.g. for safety reasons, but also in order to reduce the efforts required for exact positioning), a photogrammetrical procedure can be employed as well. Two camera systems with axes tilted towards each other could be used for this. Evaluation techniques known from industrial photogrammetry could be utilized.
Any measuring task, where the scanning element does not xe2x80x9cdisappearxe2x80x9d behind undercuts, can be resolved with two tilted cameras that e.g. xe2x80x9cfacexe2x80x9d the scanning element lengthwise or the ends of a tracer extension and shaft facing the scanning element. Employing a redundant number of cameras (e.g. three) also enables the measurement of objects with steep outlines. For the purpose of measuring in small bores, a camera can be employed that is arranged in such a way that it xe2x80x9clooks ontoxe2x80x9d the scanning element either in the scanning element""s or the tracer extension""s lengthwise direction. Generally a single camera, which has been aligned to the lengthwise direction of the tracer extension and shaft fastening the scanning element, is sufficient for two-dimensional measurement (also e.g. when measuring in bores).
According to the invention it is also possible to attach additional illuminated balls or other reticules on the shaft, which has the design of a fiber-optic light guide, record the position of these reticules especially photogrammetrically and to calculate the excursion of the scanning element accordingly. Balls represent a quite ideal, clear reticule, which one cannot find otherwise on the fiber. Good light coupling into the balls is achieved by disturbing the light guide properties of the shaft, e.g. by placing the bored volume-dispersing balls onto the shaft, i.e. the tracer extension, and gluing them onto the shaft. The volume-dispersing balls can also be glued to the side of the shaft, which also enables light coupling, provided that the shaft guides light all the way to its surface and is therefore not equipped with a casing in the glued area. A particularly high degree of precision is achieved when by trial the scanning element""s position is recorded (calibrated) as a function of the fiber position and fiber deflection (zones of the fiber at a distance from the scanning element). Here as well, the measuring of reticules that have been placed along the fiber instead of measuring the fiber itself is possible.